1. Field of the Invention
The present invention relates generally to the phosphating of metal surfaces having large areas wherein a layer of insoluble metal phosphate crystals is applied to the metal surface. More particularly, the present invention involves a non-intrusive method for monitoring and measuring the quality of such phosphate coatings during the phosphating process.
2. Description of Related Art
The treatment of metal surfaces to provide a coating of insoluble metal phosphate crystals is known as phosphating. The phosphate crystals strongly adhere to the metal surface and provide corrosion resistance and protection. Phosphate coatings are commonly used on metals as an excellent base for lacquer and paint finishes. Phosphating is commonly used in the automotive and appliance industries to provide both a paint base and corrosion resistance to metal surfaces prior to painting. Phosphating has also been used commonly in a wide variety of other industries to protect zinc, aluminum, cadmium, magnesium and their alloys.
The phosphating process usually involves dipping the metal surface into a tank containing a phosphating solution. Phosphating processes based on spraying the metal surface with phosphating solution are also commonly used. In order for the phosphate coating to provide desired corrosion resistance, it is important that the entire metal surface be covered with a uniform and unbroken layer of phosphate. Further, it is important that the quality of the phosphate coating be such that the phosphate crystals strongly adhere to the base metal and also provide a suitable surface finish to which lacquers and paints will bond.
Monitoring the quality of the phosphate coating presents many problems. These problems become especially critical when large metallic surfaces, such as car bodies and appliances, are inspected for the quality of phosphate coating. Phosphate quality is frequently not uniform over large manufactured metal surfaces. This lack of uniformity can be caused by many factors unrelated to the chemistry of the phosphate bath. Some of these factors are: non-uniform surface properties of the metal itself, non-uniform retention of drawing compounds over formed metal bodies, oil seepages from welded seams, non-uniform removal of heat scale, non-uniform cleaning of the metal, etc.
Because of the diverse causes and localized nature of problems associated with coating quality, the quality of the coating cannot be assessed by sample coupons. Non-destructive on-line quality monitoring is desirable. However, non-destructive methods such as X-ray fluorescence used on-line do not monitor the entire metal body and cannot therefore identify the location of smaller, localized quality defects on large surfaces.
Three important physical criteria of phosphate quality are coating weight, coating crystal size (or the relationship of crystal size and porosity) and coating composition (as exemplified in the technical literature by p-ratio). It would be desirable to provide a method and system for accurately monitoring the quality of phosphate coatings of large, complex industrial metal shapes and connected parts which is on-line, can detect and identify the location of localized defects as well as overall quality, and can measure coating weight, coating porosity or crystal size, and coating composition. Such an on-line monitoring method would eliminate many of the disadvantages associated with the present techniques.